A broad variety of new demands is being placed on the electromagnetic spectrum, leading to difficulty in allocating radio wave frequency bands as new kinds of equipment based on radio wave communication are developed.
Such demands provide pressure to employ progressively higher radio frequencies (e.g., &gt;500 MegaHertz) and need to utilize spectral space more efficiently. These trends create requirements for frequency selection components capable of high frequency operation and increasingly narrow passbands. Additionally needed are devices having low insertion loss coupled with improved out-of-band signal rejection, in a small form factor and with low power consumption.
Acoustic wave devices are becoming particularly important in the construction of electronic signal processing equipment, such as radios, paging devices, and other high frequency electronic apparatus, because they can be readily constructed on planar surfaces using integrated circuit fabrication techniques, are robust and compact, require no initial or periodic adjustment, and consume no static power.
A basic equation describing signal frequency f.sub.sig, acoustic wavelength .lambda., and properties of the acoustic medium is: EQU .lambda.f.sub.sig =V.sub.s, (1)
where v.sub.s represents acoustic velocity in the acoustic medium. For a given acoustic velocity v.sub.s, increased f.sub.sig requires reduced .lambda..
Wave propagating acoustic transducers rely on electrodes which are usually a fraction of a wavelength in width. Photolithographi constraints together with Eq. 1 determine an upper frequency limit by setting a lower electrode width limit. The current minimum electrode width is about one micrometer for practical mass-production equipment and techniques. This minimum electrode width sets the upper frequency limit between about one and two GigaHertz. At present, this is a frequency range of intense interest for development of new electronic products.
Control of fabrication variables, such as the ratio of refection element width to reflection element period (also known as metallization ratio), metal thickness, and the like, becomes progressively more difficult as photolithographic limits are approached, i.e., as the desired reflection element widths become smaller. This results in reduced fabrication yields for devices requiring reflection element widths at or near the photolithographic and etching limits.
A further problem lies in the fact that conventional techniques employing two reflection elements per wavelength provide only limited acoustic reflectivity per reflecting element in an acoustic device. This gives rise to reduced bandwidth and increased insertion loss.
Yet another problem encountered for surface acoustic wave (SAW) resonators results from the very narrow bandwidths of such devices, coupled with the sensitivity of the center frequency to fabrication-induced velocity variations.
Thus, SAW resonators employing increased linewidths and having improved (broader) bandwidth, or reduced Q, are extremely desirable for high frequency filtering applications.
What is needed are a device and techniques for device realization which are minimally sensitive to manufacturing variations, which maximize device performance while minimizing component size, and which are easily implemented in a fashion consistent with current acoustic device design, fabrication and use practices.